1. Field of the Invention
The present invention relates to control of a wire electric discharge machine, and more particularly to a wire electric discharge machine which prevents machining errors from being caused by wire electrode consumption due to electric discharge.
2. Description of Related Art
A wire electric discharge machine impresses a voltage between the workpiece to be machined and a wire electrode to generate an electric discharge, thereby machining a workpiece into a desired shape. The wire electrode is vertically stretched under tension and fed from a lower side to an upper side or from the upper side to the lower side at a given speed. At a position where the wire electrode faces the workpiece, an electric discharge occurs to remove workpiece materials, whereby machining the workpiece. During the machining, not only the workpiece but also the wire electrode is consumed by the electric discharge.
In case where the wire electrode is fed downwardly from the upper side to the lower side for instance, the degree of wire electrode consumption increases as the wire electrode moves downwardly. With the advance of the wire movement to the lower side, therefore, a diameter of the wire electrode decreases. On the other hand, an offset amount between a central axis of the wire and a programmed path that specifies a machining shape (i.e., the sum of a wire radius and an amount of gap between the wire electrode and the workpiece) is kept constant. Thus, the amount of gap increases toward the lower side of the workpiece by the amount of decrease of the wire diameter, so that insufficient machining is caused, resulting in a decrease in width of a machined groove and/or in a machined amount. As a consequence, straightness of shape of the machined workpiece is impaired. Specifically, as shown in FIG. 7, the width of the workpiece 1 increases toward the lower side by the amount of decrease of the diameter of the wire electrode 2. In machining of a punch shape, there occurs a case where a workpiece to be machined into a cylindrical shape is machined into a truncated conical shape, as shown in FIG. 8a. In machining of a die shape, there occurs a case where a diameter of a machined hole decreases downwardly in the direction of the wire feed, as shown in FIG. 8b. 
Nevertheless, it has not been taken any measures to prevent the influence on the machined shape by the consumption of the wire electrode in conventional wire electric discharge machines.